The basic image-forming process of silver halide photography comprises the exposure of a silver halide photographic recording material to actinic radiation (for example, light or X-rays), and the manifestation of a usable image by the wet, chemical processing of the material. The fundamental steps of this processing entail, first, treatment of the recording material with one or more developing agents wherein some of the silver halide is reduced to metallic silver. With black-and-white photographic materials, the metallic silver usually comprises the desired image. With color photographic materials, the useful image consists of one or more images in organic dyes produced from an oxidized developing agent formed where silver halide is reduced to metallic silver.
To obtain useful black-and-white images it is usually desirable to remove the undeveloped silver halide, and to obtain useful color images it is usually desirable to remove all of the silver from the photographic element after the image has been formed. In black-and-white photography the removal of undeveloped silver halide is accomplished by dissolving it with a silver halide solvent, commonly referred to as a fixing agent. In color photography the removal of silver is generally accomplishedby oxidizing the metallic silver, and dissolving the oxidized metallic silver and undeveloped silver halide with a fixing agent. The oxidation of metallic silver is achieved with an oxidizing agent, commonly referred to as a bleaching agent. The dissolution of oxidized silver and undeveloped silver halide can be accomplished concurrently with the bleaching operation in a bleach-fix process using a bleach-fix solution, or subsequent to the bleaching operation by using a separate processing solution containing a fixing agent.
It is highly desirable to process a photographic recording material as rapidly as feasible, and an accelerated process--a shortened process compared to ones known in the art--is highly desired. In particular, shortening the silver removal step, which consumes a large amount of the total process time, is an attractive manner in which to shorten the overall processing time. Juxtaposed to the desire for an accelerated process is the desire for, and the need for, photographic recording materials and processing solutions that require lower chemical usage and that generate less polluting chemical waste.
A common approach to faster processing is to desilver the photographic material with a single processing solution that accomplishes both the bleaching of silver and the fixing of silver halide. Such a solution is called a bleach-fix bath. These solutions can be useful for processing color paper materials, but they can produce poor bleaching results when color negative films or color reversal films are processed with them. Additionally bleach-fix baths cannot be recycled as efficiently because the bleaching agent in the bleach-fix bath makes it difficult to electrolytically remove silver from the bath.
A wide variety of fixing agents and silver solvents are known. Such materials form relatively stable and soluble reaction products with silver ion or silver halides. Such agents include, for example, alkali metal and ammonium thiosulfates, thiocyanate salts, sulfites, cyanides, ammonia and other amines, imides as described in U.S. Pat. No. 2,857,274, thiols as described in U.S. Pat. Nos. 3,772,020 and 3,959,362, mesoionic 1,2,4-triazolium-3-thiolates as described in U.S. Pat. No. 4,378,424 and other mesoionic heterocyclic thiolates as described in European Patent Application 431,568, thioureas, thioacids, and thioethers as described in German Offen. 2,037,684 and U.S. Pat. Nos. 2,748,000; 3,033,765; 3,615,507; 3,958,992; 4,126,459; 4,211,559; 4,211,562; 4,251,617; and 4,267,256, phosphines as described in U.S. Pat. No. 3,954,473, and concentrated halide solutions as described in U.S. Pat. No. 2,353,661.
Thiosulfate salts are generally preferred as fixing agents because they are inexpensive, highly water soluble, non-toxic, non-odorous, and stable over a wide pH range in the fixer bath. Furthermore, thiosulfate salts form very stable, water soluble reaction products with both silver ion and with silver halides.
Fixer baths containing ammonium thiosulfate are more active and fix silver halide in a photographic recording material more rapidly than thiosulfate salts of other cations, such as sodium thiosulfate or potassium thiosulfate. Adding ammonium salts to sodium thiosulfate fixer baths increases the rate of fixing. Thus, owing to its rapid fixing action, ammonium thiosulfate is widely used as a photographic fixing agent. Ammonium ion (hereinafter sometimes referred to as "ammonium") is, however, a potentially environmentally detrimental chemical. Therefore it is desirable to produce a fixing bath that has lower concentrations of ammonium, or that has no ammonium whatsoever, in order to reduce or completely eliminate its contribution to photographic effluent pollution.
Reducing the concentration of ammonium ion in the fixer bath in order to reduce pollution will result in longer fixing times for films. Fixing times could be made shorter by using a more concentrated solution of thiosulfate salt, but this increases the chemical waste and the economic cost of the process. In order to minimize wastes, it is desirable to use fixer baths with as low a concentration of fixing agent as is practicable. Since lowering the concentration of fixing agent has the disadvantage of making fixing times and overall processing times longer, there is a need for materials that can accelerate the fixing process when added to fixing solutions that contain only low concentrations of ammonium ion, or no ammonium ion at all.
It is appreciated in the art that photographic recording materials containing silver iodide and silver iodide-containing emulsions are fixed more slowly than silver bromide, silver chloride, or silver chlorobromide emulsions. Thus, there is a need for materials and methods that can increase the fixing speed of photographic recording materials containing silver iodide and silver iodide-containing emulsions, in particular.
Furthermore, it is recognized that iodide dissolved in the fixer solution also slows the rate of fixing of a silver halide photographic material. Thus, there is a need for materials and methods that can increase the fixing speed of photographic recording materials in the presence of iodide dissolved in the fixer bath.
One approach to improve photographic recording material fixability is to employ fixing accelerating agents in the fixer bath. Numerous compounds which promote or increase the rate of fixing are known which can be added to the fixing bath. Examples of such fixing promoters are described in Chapter 15 of "The Theory of the Photographic Process", 4th Edition, T. H. James, ed., Macmillan, New York, 1977. Such substances include ammonium salts, such as ammonium chloride, ethylenediamine, guanidine, and other amines as described by E. Elvegard, Photographische Industrie, 40, 249 (1942). Other amine salts and quaternary ammonium salts such as pyridinium and piperidinium salts have been reported to accelerate fixing (M. Abribat and J. Pouradier, Science and Applications of Photography (Proc. Intern. Centenary Conf., London, 1953), R. S. Schultze, ed., Royal Photographic Society, London, 1955, pp. 177-180). Thiourea has also been reported to accelerate the fixation of iodide-containing emulsions by sodium thiosulfate (J. Rzymkowski, Photographische Industrie, 1251 (1926)). However, all of these compounds have potential enviromental problems.
Thioether compounds have been reported to improve bleaching effectiveness when present in bleach or bleach-fixing baths (for example, British Patent 933,008, U.S. Pat. Nos. 3,241,966; 3,767,401; 4,201,585; 4,695,529; 4,804,618; 4,908,300; 4,914,009; 4,965,176; and 5,011,763, and unexamined Japanese Patent Application JP 02-44,355 A). These references do not disclose the use of thioethers as fix accelerators.
U.S. Pat. No. 4,960,683 of Okazaki et al describes a method for processing black-and-white photosensitive materials comprising fixing a developed black-and-white spectrally sensitized photographic material in the presence of an aliphatic thioether compound and/or a heterocyclic thiol or thiolate compound. The most preferred fixing agent is ammonium thiosulfate. The object of U.S. Pat. No. 4,960,683 is to lessen, and thereby improve, sensitizing dye stain in black-and-white materials.
U.S. Pat. No. 4,960,683 does not show that accelerated desilvering (fixing) results from the inclusion of the thioether compounds in the fixing bath. Indeed, many of the heterocyclic thiol or thiolate compounds described can, in fact, slow or inhibit desilvering. Nor is there any indication that such compounds are useful with a low ammonium thiosulfate fixing solution.
The use of ammonium thiocyanate, thiourea, or a thioether (e.g. 3,6-dithia-1,8-octanediol) to accelerate fixing by ammonium thiosulfate is mentioned in U.S. Pat. No. 5,002,861. However, as shown in this invention, acceleration of fixing by ammonium thiosulfate cannot be accomplished unless certain conditions are met. There is no indication that these compounds are useful with other thiosulfate fixing agents. Additionally, there is a need for reducing the use of ammonium ion in photographic processing solutions.
SIR H953 describes a method of processing color photographic materials in which thioether-containing compounds are present in a fixer bath which immediately follows a bleaching bath containing ammonium 1,3-diaminopropanetetraacetatoferrate(III) as the bleaching agent. The reported result of such a process is the improved bleaching of the developed silver in the photographic material. The fixing bath in this method is really a bleach-fixing bath, in which a significant amount of metallic silver bleaching occurs by the bleaching agent that is carried by the photographic material into the fixing solution during processing, and which accumulates there during continuous processing. This reference does not disclose or anticipate any improvements in the dissolution and removal of silver halide from the photographic material.
There remains a need for materials or methods that increase the speed of fixing of photographic recording materials specifically when iodide is present in the fixing solution. There remains a need for materials or methods that increase the speed of fixing of photographic recording materials specifically containing one or more iodide-containing emulsions. There remains a need for materials or methods that increase the speed of fixing of fixer baths that contain low concentrations of ammonium ion, or that contain no ammonium ion at all, so that improved processing ecology can be achieved.